This invention relates to controls for vehicle headlamp beams which automatically switch between lower and upper beams in response to the light level from oncoming vehicles. Such controls must provide sufficient sensitivity in switching from upper beam to lower beam to minimize the possibility of directing the upper beam at close range into the eyes of oncoming drivers. However, excessive sensitivity in switching from upper to lower beam may cause erratic operation or cycling between upper and lower beam due to reflected light from road signs and other objects. Both of these considerations have become more important since the introduction of halogen headlamps. Therefore, a balance or optimization of sensitivity in switching from upper to lower beam is desired whereby an upper beam will be provided up to a reasonable distance, such as 1500 to 1800 feet, from an approaching vehicle in spite of reflections from road signs and other objects and a lower beam will be provided for closer distances and held low even when the approaching vehicle lights are switched to lower beam.
In addition to the above criteria, most such automatic headlamp beam controls include an operator adjustment whereby the vehicle operator may manually adjust the switching sensitivity of the control. However, in past systems, this sensitivity control produced simultaneous changes in both the upper-to-lower and lower-to-upper switching levels. This is undesirable for the optimization described above since, at the extreme low or high settings of the driver control the sensitivity may either be so low that cycling between upper and lower beam will occur or so high that the unit will be slow to return to upper beam when the approaching vehicle has passed. This difficulty is eliminated if the driver control adjusts the upper-to-lower switching level while not affecting the lower-to-upper switching level.